The present invention relates generally to inkjet printing mechanisms, and more particularly to a ganged inkjet printhead capping system for sealing several inkjet printheads during periods of printing inactivity by coupling the printhead caps to a single, common pressure regulation chamber which accommodates for changes in altitude, barometric pressure, temperature and other environmental variations without depriming the printheads.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include an elastomeric capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have elastomeric priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick-drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality.
Thus, sealing the nozzles during periods of printing inactivity becomes even more important when using pigment-based inks, because the higher solids content contributes to the clogging problem more than earlier dye-based inks. When inkjet pens are not used during periods of printing inactivity, they are typically placed in a capping system to keep the nozzles from plugging up with dried ink. These ink plugs are caused by loss of water vapor from the printhead into the surrounding air. This loss of water vapor increases the viscosity of the ink in the nozzles until the ink becomes so thick or hard that it cannot be fired from the nozzles through energizing the associated resistor, such as in a spitting routine.
Black pens are believed to fail due to an increase ink viscosity. A slow loss of water vapor causes a thick sludge to be formed in the nozzle, which becomes so thick that the sludge cannot be evacuated by firing the associated resistor during spitting. Indeed, firing the resistor most likely creates a drying heat that actually hardens the sludge into a permanent plug. In actuality, there will most likely be a viscosity gradient through an ink plug, where the ink nearest the air interface has the highest viscosity, with the viscosity lowering toward the interior of the pen.
Color pens are believed to fail due to a hard crust being formed at the air-ink interface. The liquid behind this crusted surface has a higher viscosity than the ink stored in the reservoir, but the pen would probably fire if the hard crust was removed. Unfortunately, this hard crust still allows water to diffuse through the crust to the air surface, eventually leading to drying of the pen. This diffusion often causes a bearding phenomenon where an ink crystal grows out of the nozzle, similar to the way a salt crystal is formed through capillary pull and crystal deposits. Moreover, this water diffusion may yield vary hard plugs over the long term, such as on the order of months.
Thus far, all of the known production inkjet printers have used a diffusion path capping system. The cap has been typically implemented as an elastomeric portion that is pressed against the pen orifice plate during periods of printer inactivity. The objective of the elastomeric caps has been to control the interaction of the pen nozzles with the surrounding environment, primarily to reduce and preferably eliminate evaporation of the water from the ink composition. These elastomeric caps used diffusion paths and/or vent paths which allowed air exchange with the outside environment. These vent paths were required to regulate the pressure within the cap because any substantial increase or decrease in this pressure caused the pens to either drool (leak ink into the cap) or to deprime (ink retreating from the printhead firing chambers back into the reservoir). These abrupt pressure changes may be caused by the action of capping the pens, by changes in the temperature within the cap, by changes in the outside ambient pressure which cause a change in the mass of the air between the cap and the orifice plate, or by altitude changes due to reshipment.
For instance, the Hewlett-Packard Company""s DeskJet(copyright) 850C model color inkjet printer employed an elastomeric, multi-ridged capping system to seal the pigment based black pen. A spring-biased sled supported both the black and color caps, and gently engaged the printheads to avoid depriming them. A vent system was required including an elastomeric vent plug and a labyrinth vent path under the sled to avoid inadvertent over-pressurizations or under-pressurizations, like barometric changes in the ambient pressure or from volume changes during the capping process. Other elastomeric capping systems using a labyrinth vent paths were first introduced in the Hewlett-Packard Company""s DeskJet(copyright) 720C, 690C, 1000C and 2000C models of color inkjet printers.
Indeed, an ideal inkjet printhead capping system would maintain nozzle ink viscosity within a specified range over an incredibly long period of time, on the order of half a year, for instance. Specifically, such an ideal capping system would accomplish several objectives, including: (1) maintain a desired humidity level at the nozzles; (2) not allow the exchange of air inside the capping chamber with air from the external environment; (3) maintain a safe pressure range at the nozzles, allowing for increases or decreases in pressure as needed; and (4) accommodate ink drooling from the pen, whenever it occurs.
Thus, it would be desirable to find new ways of sealing a group of inkjet printheads which preserves pen health over longer periods of printer inactivity than the earlier capping systems.
According to one aspect of the present invention, a ganged capping system is provided for sealing nozzles of plural (more than one) inkjet printheads in an inkjet printing mechanism during periods of printing inactivity. The ganged capping system includes a cap sled which is moveable between a sealing position and a rest position. Plural caps are each supported by the sled to engage an associated one of the plural printheads when the sled is in the sealing position to form a sealing chamber surrounding the nozzles between each cap and each printhead. The ganged capping system a pressure chamber structure with an interior that defines a pressure chamber. The pressure chamber is fluidically coupled to each sealing chamber to isolate the sealing chambers from external environmental conditions when the cap sled is in the sealing position.
According to a further aspect of the present invention, a method of sealing plural inkjet printheads in an inkjet printing mechanism during periods of inactivity is provided. The method includes the step of sealing each of said plural inkjet printheads with an associated one of plural printhead caps to form a sealing chamber surrounding the nozzles between each cap and each printhead. In a coupling step, each sealing chamber is coupled to a pressure chamber defined by an interior of a pressure chamber structure, with the pressure chamber having a neutral volume before the sealing step. The method includes the step of changing the volume of the pressure chamber during said sealing step and thereafter in response to changing ambient environmental conditions external to the pressure chamber structure.
According to yet a further aspect of the present invention, an inkjet printing mechanism may be provided with a ganged capping system as described above.
An overall goal of the present invention is to provide a printhead service station for an inkjet printing mechanism that facilitates printing of sharp vivid images, particularly when using fast drying pigment based inks, co-precipitating inks, dye based inks, or ultra-fast drying inks, by maintaining pen health during long periods of printing inactivity, such as on the order of half a year.
Another goal of the present invention is to provide an inkjet printhead sealing system which maintains a desired humidity range and pressure range at the pen orifice plate, while also accommodating for inadvertent pen drooling episodes, all without allowing any exchange of the air in the cap with the ambient air.
A further goal of the present invention is to provide a method of sealing a group of inkjet printheads which preserves printhead health and provides consumers with a reliable, robust printing unit that consistently prints high quality images, even after extended periods of printer inactivity.